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Nowadays, burglar alarms have become standard equipment and common in stores, other businesses and private homes as well. There are a wide variety of options available if you have ever shopped for a home security system. However, almost all the alarm systems are actually built based on the same basic design concepts which combine electronic and electrical engineering with mechanical system together.
Burglar or safety alarms are electronic alarms designed to alert the user to a specific danger. Basically, there are sensors connected to a control unit via a narrowband radio frequency signal or low-voltage wiring which is used to interact with a response device. The opening of a door or window or detect motion via passive infrared are the most common functions of security sensors.
Burglar alarms work by using circuit principle and thus it concerns more on electrical engineering knowledge. There are two kinds of circuit alarm, closed circuit or open circuit. In a closed-circuit system, the electric circuit is closed when the door is shut which means that as long as the door is closed. The current is broken when the door is opened and triggers the alarm. Meanwhile in an open-circuit system, opening the door closes the circuit, so electricity begins to flow. In this system, the alarm is triggered when the circuit is completed. But the only drawback to this system is that intruder can easily deactivate the open circuit by simply cutting the connected wires. Therefore, closed circuits are normally a better choice than open circuits.
For the most basic design, a magnetic sensor in a closed circuit consists of a few simple components such as a battery powering a circuit, a spring-driven metal switch built into a door frame, a magnet embedded in the door, lined up with the switch, a separately-powered buzzer with a relay-driven switch. From this, we can observe that the basic idea of a closed circuit for burglar alarm is combining electronic and electrical knowledge with mechanical system. Spring-driven metal switch, relay-driven switch and buzzer are all electromechanical devices. Spring-driven metal switch works based on mechanical fundamental, Hookeââ‚¬â„¢s law; relay is an electrically operated switch and normally uses an electromagnet to operate a switching mechanism while buzzer is an electromechanical system used as alarm or timer.
The mechanism for a burglar alarm combines both electrical and mechanical engineering knowledge. When the door is closed, the magnet pulls the metal switch closed thus the circuit is complete. The current powers the relay's electromagnet, so the buzzer circuit stays open. Whenever magnet is removed by opening the door, the spring will snap the switch back into the open position. Consequence, current is cut off and closes the relay, finally sounding the alarm. This sort of system also can be built into a window. For the instance, if an intruder pushes a window open, the magnet will slide out of line with the switch, and activated the buzzer. However, there are flawed in such simple design with just a battery and buzzer because the burglar only needs to close the door again to turn the buzzer off. As a result, most of the modern burglar alarms are incorporate with control box. The control box has its own power supply and being hooked up to one or more alarm circuits. It monitors the circuits and sounds the alarm .The benefit is the control box won't cut it off once the alarm is triggered until a security code is entered at a connected keypad. Additionally, for added security, the control box is usually placed in an out of the way spot; therefore a criminal can't find it easily and attempt to destroy it.
There is also another simple burglar alarm which uses a small button as the switch. First, the button is embedded in the door frame, by closing the door pushes it in. Once the door is opened, the button is released and changing the circuit, causes sounding of the alarm. Actually all of us can simply create all sorts of alarm systems by using this basic concept of electrical knowledge and simple mechanical device. For the case intruder might break through a window; the solution is to run a current through a thin line of foil wire affixed to the surface of the glass. Once the glass is broken, it will cut off the circuit and trigger the alarm. Moreover, we can apply an open circuit design into floor mat with two metal strips spaced apart. Whenever somebody steps on mat will exert pressure on it and pushes the two metal strips together to complete the circuit. In this way, mechanical fundamentals are helping a lot because a minor mistake in calculation of pressure or space between two metal strips will fail to trigger the alarm.
It is an undeniable fact that circuit alarms are very effective for guarding the perimeter of a house, but they work poor inside a building due to the highly unpredictable motion of intruder. Motion detector could be the best mechanical device to detect the intruder who's already in the house and these basic motion detectors are fairly common nowadays. In this
field, mechanical engineering plays a vital role in such kind of deviceââ‚¬â„¢s design. Different sorts of detectors can be found in security system such as radar-based motion detector, photo-sensor motion detector, passive infrared motion detector, ultrasonic detector, microwave detector, glass break detector, smoke, heat and carbon monoxide detector.
One of the examples of radar-based motion detector is the automatic door opener. It works based on mechanical basic which is wave energy. Normally, bursts of microwave radio energy or ultrasonic sound waves will be emitted from the box above the door and then senses if that energy bounces back. The radio energy will bounce back in the same pattern if there is nobody in front of the door. But the reflection pattern definitely will be disturbed when someone comes into its range and when this happens, the motion detector will send an alarm signal to the control box.
The next simple electromechanical design is a photo-sensor motion detector which is available in many shops or shopping mall. Again, it works using mechanical concept which is characteristic of light. As usual, the motion detector sounds a chime or bell when customer enters the store. Basically, photo-sensors have two components which is a source of focused light, normally a laser beam and a light sensor. When it applies on home security system, normally we will aim the beam at the light sensor across a passageway in the house. The reason is when someone walks across the light source and the sensor, the path of the beam will be blocked temporarily, thus the sensor will record a drop in light levels and sends a signal to the control box.
Besides, another more advanced and powerful electromechanical security system is passive infrared motion detector which is one of the most common detectors found in household and small business environments due to its affordable and reliable functionality. Typically, it concerns about the heat, mechanical knowledge. This sensor could sense the infrared energy emitted by an intruder's body heat because of its favourable characteristic. It is able to distinguish if an infrared emitting object is present by first learning the ambient temperature of the monitored space and then detecting a change in the temperature caused by the presence of an object. It can verify whether an intruder or object is actually there by using the principle of differentiation, which is a check of presence or no presence. The basic idea it works is by creating individual zones of detection where each zone comprises one or more
layers which can achieve differentiation. After that, between the zones there are areas of no sensitivity or dead zones that will be used by the sensor for comparison. For example, the sensor will detect a sharp increase in infrared energy when an intruder walks into the ranges of the detector. However, almost at every environment there will always be gradual fluctuation of heat energy in certain area, therefore detectors are designed to trigger the alarm if and only if there are tremendous change in infrared energy levels.
Furthermore, ultrasonic detector transmits ultrasonic sound waves that are inaudible to humans and the frequency range is between 25 kHz to 75 kHz. Its operation method is based on mechanical theory, the Doppler shift principle, in which a change in frequency is detected due to any object motion. This phenomenon occurs when a moving object changes the frequency of sound waves around it. Theoretically, Doppler shift event will only be detected successfully when two conditions are satisfied, there must be a motion of an object either towards or away from the receiver and the motion of the object must cause a change in the ultrasonic frequency to the receiver relative to the transmitting frequency. The transmitter is emitting an ultrasonic signal into the protected area and then the sound waves are reflected by solid objects such as the floor, walls and ceiling and finally detected by the receiver. The condition is that the frequency of the waves detected by the receiver will be same as the transmitted frequency when the surfaces are stationary. For the instance, when a person or object is moving towards or away from the detector, a change in frequency will occur as a result of the Doppler principle. Such an event initiates an alarm signal. Nevertheless, this technology is considered obsolete by many alarm professionals, and is not actively installed.
In addition, microwave detector is another electromechanical device which is almost the same principal as ultrasonic detector. This device emits microwaves from a transmitter and detects any reflected microwaves or reduction in beam intensity using a receiver. The transmitter and receiver are usually combined inside a single housing, called monostatic for indoor applications, while separate housings, called bistatic for outdoor applications. For better function, this type of detector is usually combined with a passive infra red detector or "Dualtec" alarm in order to reduce false alarms. The fundamental is it operates as an active volumetric device by generating energy in the microwave region of the electromagnetic spectrum that responds to Doppler shift frequency change, frequency phase shift and motion causing reduction in received energy.
Likewise, glass break detector which is dealing with mechanical field, acoustic is useful for internal perimeter building protection. When glass breaks, it generates sound in a wide band of frequencies and it can range from infrasonic, audio band up to ultrasonic. Generally, glass break acoustic detectors are mounted in close proximity to the glass panes and listen for sound frequencies associated with glass breaking. When glass breaks, it produces specific shock frequencies which travel through glass, window frame, surrounding walls and ceiling. After that, glass break detectors will feel these shock frequencies and activate alarm condition. Typically, the most intense frequencies generated are between 3 to 5 kHz, depending on the type of glass and the presence of a plastic interlayer.
Actually all above motion detector designs can be combined in a house to offer more complete coverage and enhance safety. But, in a typical security system, the control box will not sound the alarm immediately once the motion detectors are triggered. There will be a short delay to give the homeowner some times to enter a security code that turns the system off. On the other hand, if the security code is not entered, the control box activates alarms. Obviously, the circuit design in control box extremely depends on electrical knowledge.
The next topic that will be discussed is system connection between sensors and alarm. It seems mostly associates with the electrical and electronic engineering. The trigger signal from each sensor is transmitted either through wires or wireless means. Wired systems are tamper-evident and convenient when sensors such as passive infrared motion detectors require power to operate correctly, but costly to install will be its disadvantages. Wireless systems, on the other hand, usually use battery-powered transmitters which are easier to install, but may reduce the reliability of the system if the sensors are not supervised, or if the batteries are not maintained. Also one or more wireless repeaters may be required to get the signal reliably back to the alarm panel depending on distance and construction materials. A hybrid system may utilize both wired and wireless sensors in order to achieve the benefits of both.
As well, sounding of alarm commonly deals with electrical basic. There are several things a security system might do when it detects an intruder for example, in an advanced system, the control box will be wired to several different components which may activate a siren or other loud alarm noise, flashing outdoor lights and a telephone auto-dialler.
Additionally, it is necessary for an intrusion alarm system to be deactivated or reconfigured when authorized personnel are present. This kind of circuit design concerns with programming fundamentals, both electrical and mechanical maybe involved. Ordinarily, authorization can be indicated in numerous of ways, usually with keys or codes used at the remote panel or control panel near an entry. High-security alarms may require multiple codes, fingerprint, badge, hand-geometry, or retinal scan. On the contrary, failed authorizations should cause an alarm or at least a timed lockout to prevent experimenting with possible codes for added security. Moreover, some systems can be configured to permit deactivation of particular sensors or can also be programmed to bypass or ignore individual sensors and leave the remainder of the system armed. Likewise, home and business owners can now choose a new type of keypad control panel designed to help reduce false alarms.
As a conclusion, in recent advanced technology world, electrical and mechanical engineering are interdependent in almost every aspect of our daily life. Even a small thing like burglar alarm is works by combining the knowledge of both electrical and mechanical as we can see above.